I've regularly taken WY1968 through more than nine repitches with no problems whatsoever, producing a variety of different beers (it particularly makes a really beautiful Porter).
I currently have a sixth generation ferment going with some Old Newark ale yeast happily munching away at a Burton/Barleywine.
I also have a porter currently fermenting, using my house yeast... the 8th repitch for that one.
I used to really push things and repitch an insane number of times (20+), but nowadays I pretty much stop at around 9 or 10. I never had a problem, and they seem like they can keep going, but I figure maybe I shouldn't press my luck. It's easy enough to culture up a fresh crop.